Helium Diffraction as a Probe of Structure and Proton Order on Model Ice Surfaces.

Helium diffraction has the potential to reveal the degree of proton order at an ice surface, and has been used in the past to benchmark theoretical work. We demonstrate that previous calculations do not represent the diffraction experiment to a sufficient degree of accuracy. By combining a realistic helium-water potential with quantum calculations using exact close-coupling methods we demonstrate that the scattering is strongly energy dependent. Proton order may be inferred best from selective adsorption resonances of the helium atom, which involve multiple scattering. We use the results to discuss the validity of the latest assumptions for the ice Ih surface with respect to proton ordering.